Generally, polyester resins are prepared from aromatic and aliphatic dicarboxylic acids and alkylene glycols having an optimum structure. Polyester resins have excellent physical and chemical properties, are highly soluble in general solvents, are highly flexible, possess good adhesion to a broad range of materials, have good coating workability, etc. Due to these advantages, polyester resins are used in various applications, such as for fibers, films, and adhesives.
In the preparation of a polyester resin, the proportions of raw materials present in the main chain of the final polyester resin varies depending on the reactivity of the raw materials during esterification or transesterification and on the degree of vaporization of the raw materials during polycondensation. Since the reactivity of diol components, typified by alkylene glycols, decreases in the order of primary diols>secondary diols>tertiary diols, the residual rate of diol components in polyester main chains decreases in this order. For this reason, when it is intended to synthesize a polyester resin using a secondary or tertiary alcohol, a sufficiently long reaction time is required or a very low reaction yield is obtained.
A method for synthesizing a polyester resin using an alcohol such as 1,4-cyclohexanedimethanol or isosorbide as a reactant has been known in recent years. According to this method, however, the use of the secondary alcohol makes it difficult to achieve a high reaction yield and a markedly improved degree of polymerization of the resin. That is, when isosorbide is used for the purpose of increasing the physical properties (e.g., heat resistance) of the resin, a low degree of polymerization is caused or an excessively large amount of the raw materials remain without participating in the reaction. As a result, the final polyester resin may have problems of low impact resistance or durability and very poor appearance properties.
On the other hand, it is known that various forms of metals or metal oxides can be used as catalysts for an esterification or transesterification reaction. An example of such catalysts is antimony oxide dissolved in a glycol solution. However, the use of the antimony compound brings about the formation of an insoluble antimony complex that clogs spinnerets in the resin preparation process. As a result, frequent stoppages of work are caused during resin spinning or continuous washing of the spinnerets is required.
Currently, environmental pressures and legal regulations regarding plastic resins are steadily on the rise. It is particularly important to determine whether plastic resins (such as food packaging containers) in direct contact with food contain compounds harmful to humans.
Furthermore, some already known polyester resins do not substantially lose their viscosity during hot extrusion, making it difficult to mold into final products with a uniform thickness or to manufacture large-volume or large-area products.
Thus, there is a need to develop a method for preparing a polyester resin with improved physical properties, excellent appearance properties, and good moldability into the final product, by which the efficiency of the polymerization reaction and the residual rate of raw materials in the final product can be increased.